Lithium secondary batteries, such as lithium ion batteries, boast smaller size, lighter weight and higher energy density than existing batteries. Accordingly, lithium secondary batteries are preferably used as so-called portable power sources in personal computers or mobile terminals, and as electric power sources for driving in vehicles.
Sealed lithium secondary batteries are one form of such batteries. One such battery has typically a construction wherein an electrode assembly comprising positive and negative electrodes, each provided with a mix layer that comprises an active material, is accommodated inside a battery case, together with an electrolyte (typically, an electrolyte solution), after which a lid body is fitted to close (seal) the battery case. The sealed lithium secondary battery is ordinarily used in a state where the voltage is limited so as to lie within a predetermined region (for instance, 3.0 V to 4.2 V). However, the predetermined voltage may in some instances be exceeded, and overcharge may occur, if more current than usual is supplied to the battery, for instance due to some malfunction.
Current interrupt devices (CIDs) that cut off charging current when pressure inside a battery case becomes equal to or higher than a predetermined value are one widely-known overcharge countermeasure technique. Ordinarily, a nonaqueous solvent or the like, in an electrolyte, becomes electrolyzed and generates a gas when the battery is brought to an overcharge state. On the basis of this gas generation, the current interrupt device cuts off the charging circuit of the battery, such that overcharge can be prevented to proceed any further.
Incorporating beforehand, into the electrolyte, a compound (hereafter referred to as “overcharge inhibitor”) having an oxidation potential that is lower than that of the nonaqueous solvent of the electrolyte (i.e. a lower voltage at which an oxidative decomposition reaction starts), is a known method that is resorted to when using such current interrupt device. When the battery enters an overcharge state, such an overcharge inhibitor quickly undergoes oxidative decomposition, and generates hydrogen ions (H+) as a result, at a positive electrode surface. The hydrogen ions diffuse into the electrolyte, and are reduced at the negative electrode, giving rise thereby to hydrogen gas (H2). The internal pressure of the battery rises on account of the generated hydrogen gas, and as a result, the current interrupt device can be activated quickly. As an instance of such prior art, for example Patent document 1 discloses the feature of using cyclohexylbenzene (CHB) and/or biphenyl (BP) as an overcharge inhibitor.